The present invention relates to a grinding operation, and in particular, to a method of detecting and/or preventing grind burn on a workpiece being ground.
The design and manufacture of bearings, gears, shafts and many other surface hardened components in modern automotive and aerospace industries pose significant challenges. These components require special attention in choosing the correct parameters for heat treatment as well as for subsequent machining processes. The latter, if carried out inaccurately, may reduce the surface hardness and diminish the compressive surface stresses after surface hardening. Accurate and continuous control of machining processes such as grinding is essential in today's production of these components.
Grinding is a machining process used in the manufacture of high accuracy components to achieve the required tolerance. Compared with other machining processes, grinding requires a very large energy input per unit volume of material removed. The majority of this energy is converted to heat, which is concentrated in the surface layers of the material, within the grinding zone. As such, a sharp increase in the localized temperature within the surface can occur. Gears and other components that are hardened and subsequently ground can be subjected to surface tempering of these localized areas known as “grind burns.” The severity of the damage, i.e., grind burn, will depend on the temperature the workpiece surface attained when ground. In a gear, for example, a grind burn can lower the surface hardness, lower the contact fatigue life of the gear, and cause microcracks in a burnt tooth, which negatively affects the fatigue life of the gear.
There are several factors that contribute to the generation of grind burns. Such factors can include 1) a high stock removal rate during grinding; 2) unexpected increase in stock removal from a tooth surface due to nonuniform heat treat distortion; 3) high grinding wheel hardness; 4) imbalance of grinding wheel; 5) infrequent dressing of the grinding wheel; and 6) insufficient coolant for removing generated heat. In a conventional process control method, grind burns are detected after the grinding operation. There are two primary conventional methods for inspecting a gear, for example, for grind burns: 1) a destructive method based on microhardness reading of the surface below the burnt area; and 2) a non-destructive method such as nital etching. The destructive method for inspecting gears requires the gear to be destroyed and therefore renders it unusable. This method is clearly disadvantageous because not all gears can be tested, and the gears which are not tested may suffer damage that is not detectable.
On the other hand, nital etching is currently considered the industry standard for inspecting gears for grind burns. Nital etching comprises the following steps: 1) cleaning the gear and then dipping the gear in nitric acid with 3%-5% alcohol or water; 2) rinsing the gear with water; 3) dipping the gear in alcohol; 4) bleaching the gear with hydrochloric acid in 4%-6% alcohol or water; 5) rinsing the gear again with water; 6) neutralizing the gear with an alkali solution (minimum pH of 10); 7) rinsing the gear a third time with water; 8) dipping the gear in alcohol; and 9) applying an oil with rust preventative to the gear. After the etching procedure, the gear is visually inspected for evidence of grind burns under a light source of 200 footcandles (ftc) minimum. A gear that has a grind burn can have a dark gray, blue, or black appearance, whereas a gear that is free of grind burns can have a light gray or light brown appearance. A limited amount of grind burn on a gear tooth may be acceptable, but only if the tooth is part of a non-fracture-critical gear or if the grind burn does not extend into a critical area of the tooth.
There are several disadvantages to nital etching. First, nital etching can reduce the size of the gear. For example, approximately 0.003 mm of material can be removed from the gear each time the etching process is performed. Any portion of the gear that requires a tight tolerance which should not be exposed to nital etching must be masked to avoid stock removal (which requires an additional step in the nital etching process described above). A second disadvantage with nital etching is the resulting appearance of the gear. There may be areas of discoloration on the gear as a result of nital etching. Processes for removing the discoloration may cause stock removal or surface texture changes. Another disadvantage with nital etching is corrosion of the gear. While it is possible to add corrosion protection to the gear, this requires an additional step to the above-described nital etching process. A fourth disadvantage is hydrogen embrittlement when atomic hydrogen enters the hardened steel or other alloys. Hydrogen embrittlement may cause a loss in ductility, load-carrying ability, and/or cracking. Catastrophic brittle failures are also possible. Other disadvantages with nital etching include environmental considerations, safety concerns, increased costs, and lead time. Also, the quality of the inspection of a gear or part after nital etching depends on the visual capability, skill, and awareness of the inspector performing the inspection.
In addition, not all manufactured parts are required to be inspected for grind burns using the nital etching process. According to industry standard ANSI/AGMA 2007-C00, which specifies standard procedures and requirements for the detection and classification of localized overheating on ground surfaces by chemical etch methods, there is no “specific acceptance or rejection criteria” contained therein for inspecting ground parts. In some instances therefore only a certain percentage or quantity of parts made are inspected. As such, a percentage of parts being made are never tested for grind burns.
Other non-destructive methods for detecting grind burns include Magnetic Barkhausen Noise (MBN) and X-ray diffraction. MBN measures residual stress in the gear, but this method has difficulty identifying “good quality” gears from “poor quality” gears. On the other hand, the X-ray diffraction method is expensive and time-consuming. Another detection method is to shot peen the surface of the gear. If the surface is soft, the method detects this softness due to the texture of the gear. The test is subjective, however, and relies on visual inspection for identifying grind burns.
What is needed therefore is an improved method of detecting and preventing grind burns on a ground workpiece which overcomes the disadvantages of the prior art and which can be implemented for testing all ground components being made.